The present invention relates to non-invasive methods for distinguishing bacterial infections from viral infections in humans and other animals by monitoring changes in isotopic ratios in breath.
Rapid determination of what type of infection is present in a patient complaining of symptoms associated with infection is very important in helping speed treatment, minimize the adverse effects of the infection, reduce the risk of spread of infection to others, and reducing the cost of ineffective treatment. Further, it helps avoid the risk of adverse side effects which may be inherent or associated with inappropriate treatments (e.g. if they are provided as a prophylactic measure or otherwise due to an inability to properly classify the nature of the infection at an early stage).
More recently there has been an increasing concern about the possibility of a biochemical attack by an agent that would not be known to medical personnel until several days after the attack. Where such an attack is suspected, it could well be desirable to screen a relatively large population of potential victims who may not have yet complained of symptoms, as a triage measure. In such a context it is particularly desirable to be able to spot exposure to an infectious agent very shortly after the exposure may have occurred.
There are currently a number of culturing and other techniques which are used to try to distinguish definitively between bacterial and viral infections. However, these can be time consuming, require the use of a sophisticated laboratory, be costly, and be otherwise disadvantageous. Hence, treating physicians will sometimes instead, or as an interim measure, prophylactically treat with drugs. For example, an antibiotic effective against a wide range of bacteria might be prophylactically prescribed for an infection even before it is clear that the infection is bacterial in nature.
Where the infection turns out to be viral instead, the cost of the drug will be unnecessarily incurred. Further, the patient will be exposed to the inherent side effect risks of an ineffective drug (e.g. some percentage of patients have severe allergic reactions to antibiotics).
Moreover, a decision to prophylactically treat with an unnecessary antibiotic can in some cases lead to the ineffectiveness of that antibiotic against certain future bacterial infections (a phenomenon known as antibiotic resistance development). This is particularly problematic for those bacterial infections where only a few antibiotics are known to be effective against the particular bacteria.
Researchers have previously studied the isotopic ratio of 13C/12C in human breath. In most of these experiments subjects were administered artificially labeled 13C substrates before the study began. In one such experiment the researcher studied glucose metabolism during exercise as measured by CO2 mass spectrometry after feeding 13C labeled glucose to the subjects. Similarly, in U.S. Pat. No. 6,878,550 13C-labeled CO2 was monitored following ingestion of a 13C-enriched glucose source to study diabetic indications.
Researchers have also fed 13C-labeled urea to subjects in an attempt to detect bacteria which cause ulcers. Urease activity in ulcer bacteria converts urea to carbon dioxide and ammonia. Breath samples were collected from the test subjects and analyzed for the presence of labeled isotope in exhaled carbon dioxide.
Jarvis, et al., 24 Medicine and Science in Sports and Exercise 320-326 (1992) disclosed the effect of different food matrixes labeled with 13C on breath CO2 isotopic ratios during moderate exercise, and Murphy, et al., 65 Archives of Disease in Childhood 574-578 (1990) discloses the use of isotopic ratios in breath CO2 to test for lipase activity in the human gut after feeding the subjects fat labeled with 13C. Of course, this research required the subject to be artificially dosed with an unusual isotope mix, and was not focused on testing the status of an isotopically unenriched population.
In U.S. Pat. No. 5,912,178 our laboratory reported that the catabolic state of animals (e.g. humans) consuming themselves is characteristic of the onset of infection (and certain other stresses). The disclosure of this patent, and of all other publications referred to herein, are incorporated by reference as if fully set forth herein.
In that patent our laboratory reported that even without isotope doping one could take samples from an animal of breath (and/or other specimens), evaluate certain isotope ratios (e.g. 13C/12C or 15N/14N) in those specimens, and then use information derived therefrom to determine if the animal was undergoing a catabolic state. For example, that patent describes the measurement of changes in the 13C/12C isotopic ratio of exhaled carbon dioxide using mass spectrometry.
That patent also described applications in connection with veterinary monitoring. Hence, such technology has applicability to animal agriculture as well.
U.S. Pat. No. 4,298,347 discussed a method for analyzing isotopic ratios in exhaled carbon dioxide, and that was useful in connection with our laboratory's mass spectrometry work. The method involved a solvent and an organometallic compound that reacted with gaseous carbon dioxide and formed a soluble carbonyl compound which has a unique and well separated infrared spectral peaks.
Of course, mass spectrometry equipment is relatively expensive, can require considerable training, and is often unsuitable for installation and use in small offices or in portable medical facilities (e.g. an emergency vehicle). Thus, in E. Crosson, Stable Isotope Ratios Using Cavity Ring-Down Spectroscopy: Determination of 13C/12C for Carbon Dioxide in Human Breath, 74 Anal. Chem. 2003-2007 (2002) there was a discussion of the desirability of using cavity ring-down spectrometers employing a near-IR external cavity diode laser to measure such 13C/12C ratios in human breath. It was noted that this type of equipment is more compact, less expensive, and more portable than conventional mass spectrometry equipment, and thus the use of this type of equipment could render more practical such measurements for medical diagnostic purposes. The article also noted that it was possible that such equipment could create a breath test for the presence of a particular bacteria associated with stomach ulcers if the patient ingested 13C labeled urea (a doping environment).
For further discussion of use of cavity ring-down spectroscopy to analyze stable isotope ratios in carbon dioxide see generally T. Spence et al., A Laser Locked Cavity Ringdown Spectrometer Employing An Analog Detection Scheme, 71 Review of Scientific Instruments 347-353 (2000).
While the art had previously taught that stable isotope ratios would change in the presence of stressing of the animal (such as by catabolism caused by infection), the art had not previously taught a way to distinguish bacterial from viral infection based on monitoring such ratios, much less in the context where the patient was isotopically unenriched (particularly doped with isotopes for the purpose of the experiment).
Hence, improved methods are desired for distinguishing bacterial from viral infection.